In many instances, it is desirable to cause a normally non-radioactive object to become radioactive by imbedding or applying radioactive material the normally non-radioactive object. For instance, in the case of intra-arterial stents used to prevent restenosis or reclosure of the artery subsequent to balloon angioplasty or atherectomy, it has been found that causing the stent to be radioactive irradiates the tissue in close proximity to the implantation site of the stent, thus reducing rapid tissue growth around the stent. Reducing the rapid tissue growth decreases the likelihood of restenosis.
A radioactive stent described in U.S. Pat. No. 5,059,166 to Fischell et al. (the '166 patent) shows a helical spring stent that is caused to be radioactive prior to insertion into the artery. The '166 patent discusses various techniques for causing the stent to be radioactive, including using radioisotopes in the manufacture of the stent and/or plating the stent with a radioisotope coating. Although the '166 patent generally describes a process for making the stent radioactive, it may be assumed that using radioisotopes and/or applying a radioactive coating would require handling of radioactive material in a manner which may be unacceptable for mass production purposes.
An alternative approach for creating a radioactive stent involves using a cyclotron to bombard stainless steel stents with a proton beam to produce radioactive isotopes within the stent. However, these isotopes have high-energy gamma emissions and long half-lives that make this technique impractical for humans. Gamma emissions increase the whole-body dose of radiation while having relatively little therapeutic effect on local tissue compared to the effect of beta emissions. In addition, it has been found that long half-life materials are less appropriate since optimum radioactivity-mediated inhibition is more likely to be achieved by a continuous exposure for the first few weeks following the insertion of the stent. Accordingly, a half-life of a few weeks (i.e., one to seven weeks) is deemed ideal for this purpose.
Another technique for manufacturing radioactive stents involves first implanting massive doses of .sup.31 P in titanium stents. The .sup.31 P is subsequently activated by a nuclear reactor to produce .sup.32 P. However, this technique requires up to thirty atomic percent .sup.31 P under the surface of the titanium stent, which alters the chemical composition of the alloy with unpredictable effects on the mechanical and biocompatibility properties of the stent. In addition, the titanium metal stent may have impurities which, when bombarded in a nuclear reactor, create isotopes that emit substantial gamma rays and have long half-lives similar to the isotopes in the stainless-steel stents that are bombarded in a nuclear reactor.
An article titled, "Production and Quality Assessment of Beta Emitting P-32 Stents for Applications in Coronary Angioplasty", by Janicki et al. discusses using ion beam implantation to implant radioactive .sup.32 P isotopes into a titanium stent. Although the ion beam implantation technique itself appears to result in a radioactive stent having acceptable characteristics, Janicki et al. disclose performing the ion beam implantation using radioactive cathodes that are prepared using radioactive sats which are first dissolved into liquids and then dried onto the cathode. Handling the thus-formed radioactive liquids may be unacceptable for mass production. In addition, the resulting radioactive salts that are dried onto the cathode may fall off and contaminate workers in a mass production setting.